1. Field of the Invention
This invention relates to a multilayer coating for the interior surface of a can, and more specifically, to a multilayer coating having excellent flavor retaining property, adhesion, corrosion resistance and fabricability, which is to be directly applied to the surface of a metallic can.
2. Description of the Prior Art
In the past an epoxy-type resin having superior corrosion resistance, fabricability and flavor retaining property has been used as a binder component of mainly solvent-base paints for cans. In water-base paints for cans, too, it is desired to use an epoxy resin as a binder because of its superior properties, and various proposals have been made on the use of epoxy-type resins. One example of a paint suitable for coating the interior surface of a can is a composition obtained by modifying an epoxy-type resin with a acrylic resin having a high acid value, neutralizing the excess of carboxyl groups with ammonia or an amine and dispersing the resulting modified epoxy resin in water (see, for example, Japanese Laid-Open Patent Publications Nos. 3481/1980 and 3482/1980). Another example of this type of paint is a composition obtained by graft-polymerizing an acrylic monomer containing a carboxyl group to the main chain of an epoxy resin, and dispersing the graft polymer in water using a basic compound such as ammonia and an amine (see Japanese Laid-Open Patent Publication No. 3482/1980).
The paints for coating the interior surface of a can containing solvent-base epoxy resins as binders give superior film properties, but it is still desired to improve, the flavor retaining properties, adhesion, corrosion resistance and fabricability of the resulting coatings.
On the other hand, a coating composition comprising conventional aqueous dispersions of epoxy resins requires the use of a curing agent such as an amino resin or a phenolic resin because curing of the coating in the drying step is not sufficient. However, the use of these curing agents renders the flavor retaining property and adhesion (water-resistance adhesion) of the coating from the resulting aqueous coating composition insufficient, and this aqueous coating composition is unsuitable for application to the interior surface of a can.
Thus, conventional paints for coating the interior surface of a can cannot satisfy all of the required properties such as flavor retaining properties, adhesion, corrosion resistance and fabricability.
In view of the above state of the art, the present inventors repeated extensive investigations in order to develop a coating for the interior surface of a can which has superior flavor retaining properties, water-resistance adhesion, corrosion resistance, fabric-ability ability and hygienic property, and consequently found that the aforesaid problems can be solved by forming the coating in a multilayer and allotting the function of the coating to the individual layers.